{ 1 , 2 } , { 4 , 5 , 6 } , { 9 , 1 0 , 1 1 , 1 2 } , { 1 6 , 1 7 , 1 8 , 1 9 , 2 0 } , …
The first n positive integers are partitioned into sets as described above.
Let S n be the sum of the elements of the set at n th position. Find the value of 3 S 5 1 − S 5 0 S 5 0 .
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same method as of me ! :)
Same as @Parth Lohomi
I instead used the same method but made a mistake in the calculation
First term of each set is square of the number of the set so the first term of 50th set will be 2500
And last term of the nth set is given by n(n+1) so the last term of the 50th set will be 2550
All the terms will be in A P
thus the sum of the terms of the 50th set will be 128775
Similarly doing in the 51th set the first term will be 5 1 2 =2601
And the last term 2654
And all these are in AP THUS THE SUM OF ALL THE terms will be 136578
So we have got S(50) and S(51) on putting in the question we get s51-s50=7803
7 8 0 3 =51 3 ----------(S51-S50)
So final answer is
5 1 1 2 8 7 7 5 3 * 3 = 2 5 2 5
This is our answer
exactly my solution. :D
Observe sets.....each set has one element more than its position and highest value is position * no of elements.... So s50 has 51 elements starting from 2500 to 2550 and s51 has elements starting from 2601 to 2652(51*52)....find both sums using arithmetic progression and substitute. And is 2525.
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For the n t h set we see that it contains the numbers from n 2 to n 2 + n
Therefore the sum of the numbers in the n t h set is:
n 2 + ( n 2 + 1 ) + ( n 2 + 2 ) + . . . + ( n 2 + n )
= n 2 ( n + 1 ) + 2 1 n ( n + 1 )
= 2 1 ( n + 1 ) ( 2 n 2 + n ) = 2 1 n ( n + 1 ) ( 2 n + 1 )
Therefore, we see that ( 3 ) S 5 1 − S 5 0 S 5 0
= ( 3 ) 2 1 ( 5 1 ) ( 5 2 ) ( 1 0 3 ) − 2 1 ( 5 0 ) ( 5 1 ) ( 1 0 1 ) 2 1 ( 5 0 ) ( 5 1 ) ( 1 0 1 )
= 3 2 1 ( 5 1 ) ( 5 2 ∗ 1 0 3 − 5 0 ∗ 1 0 1 ) 2 1 ( 5 0 ) ( 5 1 ) ( 1 0 1 )
= 3 2 1 ( 5 1 ) ( 5 3 5 6 − 5 0 5 0 ) 2 1 ( 5 0 ) ( 5 1 ) ( 1 0 1 )
= 3 2 1 ( 5 1 ) ( 3 0 6 ) 2 1 ( 5 0 ) ( 5 1 ) ( 1 0 1 )
= 3 ( 5 1 ) ( 5 1 ) ( 3 ) 2 1 ( 5 0 ) ( 5 1 ) ( 1 0 1 )
= 5 1 2 1 ( 5 0 ) ( 5 1 ) ( 1 0 1 )
= 2 5 ∗ 1 0 1 = 2 5 2 5